Since a Ziegler-Natta catalyst widely applied for the existing commercial process is a multisite catalyst, the molecular weight distribution of the produce polymer is broad, and the chemical composition distribution of the comonomers is not uniform, and thus, there is a limit in achieving desired properties.
To the contrary, a general metallocene catalyst is a single site catalyst having one kind of active site, and the polymer produced using the same has narrow molecular weight distribution, and the molecular weight, stereoregularity, crystallinity, and particularly, reactivity of comonomers may be substantially controlled according to the structures of the catalyst and ligand.
The metallocene catalyst system consists of a main catalyst including a transition metal compound, particularly a Group 4 metal of the Periodic Table as a main component, and an organometallic compound cocatalyst including Group 13 metal such as aluminum as a main component. As the catalyst has single site, polymer such as polyolefin having narrow molecular weight distribution may be prepared using the same.
The molecular weight and the molecular weight distribution of polyolefin are important factors for determining physical properties, flow influencing on the processability, and mechanical properties of the polymer. To prepare various polyolefin products, it is important to improve melt processability through the control of molecular weight distribution. Particularly, in the case of polyethylene, toughness, strength, environmental stress resistance and the like are very important. Thus, a method of improving the mechanical properties of high molecular weight resin and the processability of low molecular weight resin by preparing polyolefin having bimodal or wide molecular weight distribution is suggested.
Meanwhile, as a support for a supported catalyst, silica is generally used, and a cocatalyst methylaluminoxane (MAO) and at least one organometallic catalyst (for example, a metallocene catalyst) are supported on the support to prepare a supported catalyst.
However, this method has a limitation in broadening molecular weight distribution because only one active site exists, and the utilization of siloxane groups on the surface is lowered, and thus, when polyolefin is prepared using the same, catalytic activity is low and olefin polymer having high molecular weight cannot be prepared.
Accordingly, there is a demand for the development of a novel catalyst that has improved catalytic activity and can prepare polyolefin having wide molecular weight distribution.